1. Field of the Invention
The present invention generally relates to the field of implantable heart stimulation devices, such as pacemakers, and similar cardiac stimulation devices that also are capable of monitoring and detecting electrical activities and events within the heart. More specifically, the present invention relates to an implantable medical device and a method for monitoring ventricular synchronicity of a heart.
2. Description of the Prior Art
Implantable heart stimulators that provide stimulation pulses to selected locations in the heart e.g. selected chambers have been developed for the treatment of cardiac diseases and dysfunctions. Heart stimulators have also been developed that affect the manner and degree to which the heart chambers contract during a cardiac cycle in order to promote the efficient pumping of blood.
Furthermore, the heart will pump more effectively when a coordinated contraction of both atria and ventricles can be provided. In a healthy heart, the coordinated contraction is provided through conduction pathways in both the atria and the ventricles that enable a very rapid conduction of electrical signals to contractile tissue throughout the myocardium to effectuate the atrial and ventricular contractions. If these conduction pathways do not function properly, a slight or severe delay in the propagation of electrical pulses may arise, causing asynchronous contraction of the ventricles which would greatly diminish the pumping efficiency of the heart. Patients, who exhibit pathology of these conduction pathways, such as patients with bundle branch blocks, etc., can thus suffer from compromised pumping performance. For example, asynchronous movements of the valve planes of the right and left side of the heart, e.g. an asynchronous opening and/or closure of the aortic and pulmonary valves, is such an asynchrony that affects the pumping performance in a negative way. This may be caused by right bundle branch block (RBBB), left bundle branch block (LBBB), or A-V block. In a well functioning heart, the left and right side of the heart contract more or less simultaneously starting with the contraction of the atria flushing down the blood through the valves separating the atria from the ventricles, in the right side of the heart through the tricuspid valve and in the left side of the heart through the mitral valve. Shortly after the atrial contraction the ventricles contract, which results in an increasing blood pressure inside the ventricles that first closes the A-V plane valves and after that forces the outflow valves to open. In the right side of the heart it is the pulmonary valves that separate the right ventricle from the pulmonary artery that leads the blood to the lung, which is opened. In the left side of the heart the aortic valve separates the left ventricle from the aorta that transports blood to the whole body. The outflow valves, the pulmonary valve and aortic valve, open when the pressure inside the ventricle exceeds the pressure in the pulmonary artery and aorta, respectively. The ventricles are separated by the intraventricular elastic septum. Hence, for a well functioning heart a substantially synchronous operation of the left and right hand side of the heart, e.g. a synchronous opening and/or closure of the aortic and pulmonary, is of a high importance.
When functioning properly, the heart maintains its own intrinsic rhythm. However, patients suffering from cardiac arrhythmias, i.e. irregular cardiac rhythms, and/or from poor spatial coordination of heart contractions often need assistance in form of a cardiac function management system to improve the rhythm and/or spatial coordination of the heart contractions. Such systems are often implanted in the patient and deliver therapy to the heart, such as electrical stimulation pulses that evoke or coordinate heart chamber contractions. Thus, implantable heart stimulators that provide stimulation pulses to selected locations in the heart e.g. selected chambers have been developed for the treatment of cardiac diseases and dysfunctions. Heart stimulators have also been developed that affect the manner and degree to which the heart chambers contract during a cardiac cycle in order to promote the efficient pumping of blood.
In particular, various prior art procedures have been developed for addressing disorders related to asynchronous function of the heart. For instance, cardiac resynchronization therapy (CRT) can be used for effectuating synchronous atrial and/or ventricular contractions. Furthermore, cardiac stimulators may be provided that deliver stimulation pulses at several locations in the heart simultaneously, such as biventricular stimulators. For example, patients with heart failure symptoms and dyssynchronized cardiac chambers are often offered such a CRT device that synchronizes the right and left ventricle to obtain an improved cardiac functional performance and quality of life. The CRT settings should be optimized in terms of VV interval and AV interval for optimized pumping performance. In the majority of CRT patients this optimizing of CRT parameters is normally performed at implant and perhaps at one regular follow-up. Ideally, this optimization should be performed more frequently to match the actual need of the patient.
In a healthy heart the sinus node, situated in the right atrium, generates electrical signals which propagates throughout the heart and control its mechanical movement. Some medical conditions, however, affect the relationship between the electrical and mechanical activity of the heart and, therefore, measurements of the electrical activity only cannot be relied upon as indicative of the true status of the heart or as suitable for triggering stimulation of the heart.
Hence, asynchronous depolarization of the ventricles results in asynchronous myocardial contractions with regional dyskinetic cardiac tissue. The cardiac performance is very sensitive to small asynchronous cardiac movements, as the overall heart cycle is disturbed. One consequence is that not only is the systolic part of the heart cycle is less effective, but the diastolic phase (the filling phase) of the heart cycle may also be greatly tampered.
Consequently, there is a need within the art of methods and devices for obtaining accurate and reliable signals reflecting different aspects of mechanical functioning of the heart, and, in particular, reflecting the synchronicity or dyssynchronicity of the functioning of the ventricles. Impedance measurements, e.g. of the intracardiac impedance, has been shown to provide reliable information regarding the mechanical functioning of the heart. For example, through the impedance measurements, blood volume changes are detectable. Blood has a higher conductivity (lower impedance) than myocardial tissue and lungs. The relationship between the impedance-volume is inverse, the more blood—the smaller impedance. For example, at maximum ventricular filling after atrial contraction, in end-diastole just prior to ventricle contraction, the intracardiac impedance attaints its minimum amplitude during the course of the cardiac cycle.
One phenomenon or characteristic feature that has been observed in the diastolic intracardiac impedance is the so called notch, or intracardiac notch. This feature manifests as a consistent change in the diastolic intracardiac ventricular impedance slope after rapid ventricular filling at the change to slow filling. It has been shown that this feature can be used for diagnostic purposes, see “Studies of changes in volume in right ventricle with electrical bio-impedance”, K. Järverud, Licentiate Thesis, Karolinska Institutet, Stockholm, 2002. The notch is defined as the first positive slope on the negative diastolic impedance slope between end of T-wave and P-wave. However, a slope change immediately at the end of T-wave is not considered to be a notch
In the prior art, the diastolic notch has been used for diagnostic purposes. For example, in U.S. Pat. No. 7,082,329, the occurrence of the notch in the impedance signal coincident with the entry of blood into the ventricles is monitored to detect signs of disturbed relaxation patterns of the heart. The time derivative of the intra-cardiac impedance signal is calculated and a loop is generated by the calculated time derivative values for each cardiac cycle. The generated loop is compared with a loop template representing a normal loop for the patient to identify deviations in the loop from normal deviations in timing and shape of the loop. Furthermore, in U.S. Pat. No. 7,190,996, the notch is used for early detection of ischemic heart disease. The occurrence of the notch in the impedance signal coincident with the entry of blood into the ventricles is monitored and a measured post-notch impedance curve is compared with a stored predetermined reference impedance curve template to detect an ischemic heart disease from the result of the comparison. Thus, the intracardiac impedance and characteristic features of the impedance have been used within the art for different diagnostic purposes. Even though the intra-cardiac impedance notch has been used to monitor and detect certain cardiac deficiencies such ischemic heart disease, there remains a need for an efficient and accurate parameter for monitoring and detecting disturbances in the filling pattern of the ventricles and the synchronicity in the functioning of the ventricles. On the other hand, the intracardiac impedance signal has been used within the art to monitor the synchronicity of the ventricles and to optimize the functioning of the ventricles. For, example, in US 2007/0066905, a system for optimizing a cardiac synchronization based on measured impedance signals is shown. In one embodiment of the system, the left ventricular impedance is measured, which reflects the contraction and expansion of the left ventricle. The obtained impedance signals are used to compute the impedance-indicated peak-to-peak volume indication of the left ventricle and/or an impedance-indicated maximum rate of change in the left ventricular volume. These parameters are then used to control a cardiac resynchronization. In US 2007/0271119 a similar optimization system is described. In U.S. Pat. No. 7,330,759, a cardiac pacemaker for bi-ventricular stimulation where impedance signals is used to obtain a synchronization of the left and right ventricles is shown. In particular, the second derivative of the intracardiac impedance pattern of a cardiac cycle is determined and maximized. This is based on the assumption that the intracardiac impedance pattern respectively reflects the volume of blood in a heart, the maximum acceleration to which the blood is subjected to in the heart is to be gauged from the maximum of the second derivative of that intracardiac impedance pattern, which value is correlated to contractility of the left ventricle. These parameters of the impedance signal used for the optimization is dependent on the physiological system including, inter alia, the heart and the vascular system which, for example, may entail that a response to a change of the stimulation parameters in terms of a change of a monitored parameter will be able to detect with a delay. This may, for example, lead to an overcompensation of the stimulation parameters. Furthermore, it cannot be ascertained that the monitored parameters reflect only the hemodynamical performance of the heart, which, in turn, may lead to a stimulation parameter setting that in the long-term is not optimal with respect to the hemodynamic performance of the heart.
In summary, the prior art presents a number of approaches to monitor and detect different cardiac deficiencies based on intracardiac impedance and certain morphology features of the intracardiac impedance. However, there is still a need of a reliable parameter that provides accurate and reliable information of the mechanical functioning of the heart that can be used to monitor and detect a dyssychronism in the functioning of the ventricles during the filling phase. Moreover, in order to be able to optimize the functioning of the heart it is also of paramount interest to obtain information that may enable a fast and reliable optimization of the hemodynamic performance of the heart and, in particular, a fast and reliable synchronization of the ventricles to obtain a coordinated filling phase of the ventricles.